With continuing improvements in the packaging density and speed of operation of electronic data processor circuit components, the application of digital signal processing to a wide variety of signal processing applications has also continued to expand. In multisensor environments, such as those employed for imagery signal generation, statistical signal processing schemes are employed to enhance the electrical signals that are representative of a desired target optical image and to suppress unwanted signals (noise) which represent background clutter. In such target detection systems, large contrast changes are often employed to indicate where to segment potential targets from background. If, in an effort to eliminate noise, a low-pass filter is used for preprocessing, there is a blurring of the edges of potential targets that reduces the sharpness of the contact changes, which could result in a target being missed.
To avoid this problem there have been proposed signal processing systems which employ median filters as a basic tool for target/background separation. The median filter operates to extract a median electrical amplitude value from a set of electrical amplitude values which constitutes an image, or some portion (window) thereof, as viewed by the opto-electronic transducer array of the optical imaging system. Typically, this type of imaging system produces a filter image of the target by eliminating or blanking most electrical noise which has a value significantly different than the median value established by the median filter. Contrasted to the low-pass filter approach referenced above, the median filter preserves image edges while eliminating noise spikes.
Unfortunately, the actual filtering procedure required to implement an exact median filter has been very time consuming. With real-time data rates of 10.times.10.sup.6 pixels/sec., it has often been necessary to define a specifically tailored architecture to handle the problem. One-dimensional median filters have been proposed which employ sorter networks comprised of comparators for achieving processing in real-time. The principal motivation for using a separable median filter is that for a small number (n) of inputs (e.g. n=5) the number of comparators is reasonable and the resulting structure of such a two dimensional median filter is not too complex. However, when the number of inputs increases substantially (e.g. n=25), the number of comparators becomes large and the resulting structure is both complex and unmodular.
An example of a system employing a median filter for such a one-dimensional approach is described in the U.S. patent to Bluzer U.S. Pat. No. 4,135,248. This patented system may be used to extract running medians from a data stream in order to remove undesirable noise. To this end the digital input data is converted to base-one coded data, with each data element being entered one at a time in the processing filter. Disadvantageously, because of the use of base-one coded data, the patented system requires extremely large data busses, making it implementationally impractical and therefore unsuitable for handling high resolution multidimensional imagery data.